This “megastudy” will simultaneously test different tactics designed to increase Covid-19 booster uptake in the United States. Only half of eligible Americans have received their booster, indicating the need for tactics that can effectively encourage vaccination to counter misinformation, lack of motivation, and logistical barriers. The megastudy will be designed by an interdisciplinary team, which will develop about 10 interventions to encourage vaccination, and simultaneously evaluate their efficacy among at least 500,000 people. This megastudy aims to identify which tactics effectively increase vaccinations overall and which tactics work best for whom (e.g., based on age, gender, race, etc.), which could help address disparities in vaccination rates across different demographic groups. The insights gleaned from this megastudy can be incorporated into vaccine promotion campaigns to increase vaccinations at scale.